The present invention pertains to an inkjet printer for jetting ink that is substantially free of solvent, the printer comprising a printhead having an ink chamber with an ink inlet and an ink outlet, an ink supply reservoir in fluid connection with the chamber via the ink inlet, an electromechanical transducer in operative connection with the chamber for generating pressure waves therein, and a heater for substantially, uniformly heating the ink in the ink chamber, wherein the ink inlet comprises a constricting element.
Such an inkjet printer is known from U.S. Pat. No. 4,418,355 (DeYoung, 1983). This printer is designed for jetting inks that are substantially free of solvent, i.e., inks that dry or harden on the receiving medium without the need of large amounts of solvent to evaporate from the jetted ink. Typically these inks contain less than 10% of material that is not included in the ultimate dried ink. Developments in the field of these inks has resulted in inks that contains less than 5% or even less than 2% (ultimately approaching zero %) of material that will not be included in the dried ink. Hot melt inks and UV curable inks are typical examples of such inks. In the rest of this description, these inks will be referred to as solvent free inks.
Solvent free inks typically have a viscosity that is substantially higher than the viscosity of solvent inks. In order to be able and jet small drops of these inks out of the outlet (nozzle) of the ink chamber it is therefore required that the ink is heated to an elevated temperature. In order to provide for a stable jetting process, the inkjet head comprises a heating element for substantially uniformly heating the ink in the ink chamber. This is in complete contrast with the known bubble jet printheads which have heaters for locally heating the ink in the chamber. Such local heating may give rise to temperature gradients in the chamber itself amounting up to 40° C. In the head as known from the prior art, the temperature gradient in an ink chamber will be less than 10° C. In equilibrium circumstances this will be even less than 5° C., and most probably even less than 2° C.
As apparent from FIG. 3 of the above mentioned U.S. patent, the ink chamber is connected to an ink reservoir via an inlet comprising a constricting element. In this way, it is substantially prevented that pressure waves generated by actuating the electromechanical transducer (see FIG. 1), propagate via the reservoir to neighboring ink chambers. Such propagation induces cross-talk and produces print artefacts.
The known printhead however has an important disadvantage. Due to the fact that solvent free inks have a relatively high viscosity (even at the operating temperature of the printhead these are typical 10-15 mPa·s), the restriction in the inlet constitutes an inherent high resistance against free flow of ink from the reservoir to the ink chamber. Therefore, the restriction is bound to certain minimum dimensions depending among other things on the actual viscosity of the ink and the driving frequency of the electromechanical transducer. This means that the resistance against propagation of pressure waves is not optimal. When the integration density of the nozzles is made higher, and even more so, when the driving frequency becomes higher than 5 kHz, this disadvantage becomes even more pronounced.
Accordingly, it is an object of the present invention to overcome or at least mitigate this problem. To this end, an inkjet printhead has been developed, wherein the constricting element is such that the pressure drop over the constricting element in the direction from the reservoir to the chamber is smaller than the pressure drop over said element in the opposite direction for the same net fluid flow and wherein the ratio of the length of the constricting element and the mean diameter of this element is less than 10.